Both Yosemite and Sequoia & Kings Canyon are open year-round.
Avoid holiday weekends in spring and summer if possible -- park campgrounds are usually full June through August -- and expect some crowds in late spring and early fall as well. Winter is a great time to visit Yosemite -- not only is the park virtually empty, but there are a number of activities, from downhill skiing and sledding at Badger Pass, to cross-country skiing, snowshoeing, and even ice skating. Keep in mind that the high country along Tioga and Glacier Point roads is inaccessible to motor vehicles from mid-fall to early June, when weather closes the roads.
Climate
The climate at both Yosemite and Sequoia & Kings Canyon varies considerably, depending on the region of the park. A good rule of thumb is to remember that the higher you go, the cooler it gets. So pack a parka on any trip that climbs above the valley floor or ventures into the backcountry.
In summer, Fahrenheit temperatures at lower elevations (such as Yosemite Valley) can climb into the 90s (30s Celsius) and higher, and plummet into the 50s (10s Celsius) at night. Afternoon temperatures average in the 60s and 70s (10s-20s Celsius) in spring and fall, and again, evenings are usually cool. Afternoon showers are fairly common fall through spring. Winter days average in the 40s and 50s (5-10s Celsius), and it seldom drops below zero (-17°C), although much of the land above 5,500 feet is buried beneath several feet of snow.
The high country gets up to 20 feet of snow half the year, so visitors should be experienced in winter travel. November through March, it is wise to expect snow and be prepared. Remember, particularly wet winters lead to incredibly stunning and powerful spring waterfalls, especially in Yosemite.
